A need exists to measure and monitor voltages in many circuits and applications. For example, a circuit may monitor a supply or signal voltage level to determine specific operating conditions, such as may trigger a shutdown during low-power conditions or a specific circuit operation in response to a predetermined signal level. In one common scenario, a voltage to be monitored is a relatively high voltage, and the monitoring is performed using a circuit that either cannot operationally monitor a voltage of such a magnitude or which is not desirable to be designed to operate in such a manner as to directly monitor a voltage of such a magnitude. For example, direct monitoring of a voltage magnitude of several hundred volts may be desired, but such could destroy many integrated circuits that typically can withstand voltage magnitudes of no greater than tens of volts.
FIGS. 1A and 1B depict typical embodiments of resistive voltage-divider circuits 20, 22 in which a relatively high input voltage is monitored using the resistive voltage divider to produce a relatively low output voltage having a magnitude that is a known function of the input voltage. In FIG. 1A, the input voltage VI_1A appears between first and second input terminals VINA, VINB of the voltage divider 20, i.e., VI_1A=VINA−VINB. The voltage divider 20 includes first and second resistors RA, RB, and produces an output voltage VO_1A between a first and second output terminals VOUTA, VOUTB, i.e., VO_1A=VOUTA−VOUTB, having a value determined by the following equation: VO_1A=VI_1A*RB/(RA+RB). In the case that a large degree of voltage division is desired (e.g., in order to monitor a very large voltage using a circuit designed to run on an ordinary supply voltage), the ratio of RA to RB can be made relatively large.
The resistive voltage divider 22 of FIG. 1B is similar to that of FIG. 1A, and includes first, second and third resistors RC, RD, RE arranged in a resistor string between first and second input terminals VINC, VIND, and receives an input voltage VI_1B applied between both ends of the resistor string, i.e., VI_1B=VINC−VIND. The voltage divider 22 of FIG. 1B produces an output voltage VO_1B at first and second output terminals VOUTC, VOUTD positioned about the second resistor RD in the three-resistor resistor string, i.e., VO_1B=VOUTC−VOUTD, having a value determined by the following equation: VO_1B=VI_1B*RD/(RC+RD+RE). As with the embodiment of FIG. 1A, in the case that a large degree of voltage division is desired, the ratio of RC and RE to RD can be made relatively large.
A major problem exists, however, with the embodiments of both FIGS. 1A and 1B. In both cases, when implementing a large degree of voltage division, the output voltages are a function of the ratio of a large resistance value to a small resistance value (or a similar mathematical quantity). Thus, the accuracy of the correspondence of the output voltage to the input voltage is a function of the accuracy of this ratio, and therefore to effectuate an accurate known degree of voltage division, this ratio needs to have an accurate known predetermined value. However, manufacturing a resistive divider that achieves an accurate ratio between large and a small resistance values can be difficult and expensive. In particular, it is impractical and inefficient (from a chip-area perspective) to provide very large resistors in an integrated circuit. Alternatively, it is difficult and expensive to produce and match external resistance values to either that of other external resistors or to integrated resistors. It is also difficult to maintain the accuracy and stability of a ratio of two very different resistance values over temperature variations. Thus, there is a need in the art for a circuit and a method for measuring voltages that does not depend on the predetermined accuracy of a ratio of a relatively large resistance value to a relatively small resistance value.